Emulation
Amiga Computers: UAE
UAE is a mostly complete software emulation of the hardware of the Commodore Amiga 500/1000/2000.

Commodore Computers: VICE
The current version emulates the C64, the C128, the VIC20, all the PET models (except the SuperPET 9000, which is out of line anyway), the PLUS4 and the CBM-II (aka C610).

Macintosh: Basilisk II
Basilisk II is an Open Source 68k Macintosh emulator. That is, it allows you to run 68k MacOS software on your computer, even if you are using a different operating system.

Free OSes
6502-Based: GeckOS/A65 Operating System
OS/A65 is a full-featured Multitasking/Multithreading operating system for the 6502. It is preemptive and implements some Unix-like features, like signals, semaphores, relocatable fileformat, standard library, internet support via a kind of simplified sockets and last but not least virtual consoles.

Educational: GeekOS
GeekOS is in a very early stage of development, so don’t expect it to do anything useful. However, if you’re interested in operating systems development and are looking for a simple OS kernel to experiment with, GeekOS might be what you’re looking for.

Historically Significant: Genera OS
This was the operating system that ran on the Symbolics Lisp Machines. It was reported to have a simply awe-inspiring development environment. It is still in use in animation houses and as a development environment.

Independant: NewOS
NewOS is a open source operating system with an emphasis on design and portability. It is largely implemented in C/C++, with a small amount of assembly. Note: NewOS is used as the basis for Danger’s Hiptop device.

nix: NetBSD
NetBSD is a free, secure, and highly portable Unix-like Open Source operating system available for many platforms…

Programming Language Oddities
Minimalist: Brainfck
Müller’s goal was to create a simple Turing-complete programming language which could be implemented with the smallest possible compiler. Some interesting projects have come out of Brainf*ck, namely: a Huffman coded version named Spoon and a BF computer.

More to come…
-m